1. Field of the Invention
The present invention relates to a preform as a basis of a composite material, such as aluminum-based composite material (MMC), and a production method therefor.
2. Description of Related Art
For the purpose of dispersing a reinforcement agent (herein referred to as a reinforcement) throughout a composite material uniformly and so on, in a conventional art, a preform of a predetermined shape is produced, and it is impregnated with melted metal of a matrix-type metal by way of a forging method and an infiltration method, so as to produce a billet.
As a production method for the above-mentioned preform, a wet method is commonly used as shown in FIG. 4. Namely, water, reinforcement (ceramic fiber) and binder are prepared, and after being mixed together and adjusted, the mixture is processed by press-forming. Next, after being removed from the press, the press-formed product is dried so as to form the preform.
The wet method mentioned above, however, has problems in that the number of processes is large, and therefore the time for production is long. Further, the preform is easily broken (has high fragility), for instance, when it is removed from the die, or during the drying and baking processes. Furthermore, since water is required for the wet method, if a process for impregnating the preform with a melted metal is located close to the process for producing the preform, there must be provided means for avoiding contact between the melting metal and the water, and therefore, the wet method comprises obstacles for industrially performing the production processes from the preform to the billet on an in-line process.
Moreover, with the wet method, metal powder, which is added for obtaining a composite material of high strength and high functionality, is oxidized. As a result, it is sometimes impossible to obtain satisfactory results.
Further, according to Japanese Laid-open Patent No. Hei 5-255776 (1993) and Japanese Laid-open Patent No. Hei 6-192765 (1994), etc., there has been proposed a dry method.
In this dry method, ceramic reinforcement and inorganic binder mainly containing SiO2 are mixed together, and after being formed by applying pressure, the mixture is heated.
Further, a dry method using an organic binder has been disclosed in Japanese Laid-open Patent No. Hei 8-53724 (1996).
In this dry method, in particular in a case where the organic binder is used, the mixture must be heated to a high temperature (600xc2x0 C. to 1,100xc2x0 C.) after being formed by the application of pressure thereto. Further, in the conventional dry method using organic binders, it is necessary to add a large amount of binder into the reinforcement. These are all problems.
An object, in accordance with the present invention, for resolving the drawbacks in the conventional art mentioned above, is to provide a preform and a production method therefor, in which a preform can be obtained by the dry method without using water, which preform has sufficient strength and uses less binder.
Namely, in a preform, in accordance with the present invention, fibrous or grain-line particles in a ceramic reinforcement are bonded to one another into a polycondensation body produced by the polycondensation of a binder of the silanol group, thereby achieving strength higher than 200 kg/cm2, being sufficient for the handling thereof.
The ceramic reinforcement mentioned above can be any one containing fibrous, whisker-like, or granular particles therein, and as an example, aggregate alumina can be cited. The aggregate alumina comprises alumina particles of 1-2 xcexcm aggregating to secondary particles of sizes from several tens of xcexcm to several hundreds of xcexcm, and has a very large specific surface. In accordance with the present invention, it is possible to adhere the particles having a large specific surface together (porous body).
Further, the preform can be improved in the characteristic values thereof, by adding thereto metal powder of Fe or Mg or the like.
Moreover, in accordance with the present invention, there is provided a method for producing a preform, comprising the steps of:
mixing a binder of a silanol group with fibrous or granular particle-containing ceramic reinforcement in a ratio which is greater than 0.5 wt % and less than 2.0 wt %;
forming the mixture obtained in the above step and polycondensating the binder of silanol group through friction between the particles of the ceramic reinforcement and the binder of silanol group during the forming; and
bonding the particles of the ceramic reinforcement to one another in a polycondensation body produced by said polycondensation step.
As the silanol group binder, a mixture of silanol and siloxane, or polymethylsilsesquioxane is appropriate. A chemical formula is shown below: 
wherein R: xe2x80x94CH3 or xe2x80x94H
In the case where the mixture of silanol and siloxane is used as the binder, as is shown in FIG. 1, polymethyl siloxane having a molecular weight from 1,000 to 3,000 is produced by the polycondensation between the silanol and the siloxane in a region of low temperature, and thereby the polymethyl siloxane bonds the particles of the reinforcement (Al2O3) to one another. Here, the region of low temperature indicates the region of temperature at the time when the preform is formed, and is a temperature approximately in the range of 40xc2x0 C. to 100xc2x0 C.
However, if the formed preform is impregnated with melting metal, the polymethyl siloxane is heated to be changed into SiO2.
FIG. 2 is a graph showing the relationship between the amount of binder added and the strength of the preform when the polymethylsilsesquioxane is used as the silanol group binder. As is apparent from this graph, taking into consideration the handling characteristics, such as setting the preform into a die after the forming thereof, it is necessary that the preform has a strength higher than 200 kg/cm2, and it is apparent that the polymethylsilsesquioxane should be greater than 0.5 wt % with respect to the reinforcement for obtaining such a strength.
On the other hand, since the strength of the preform is not additionally improved even if the amount of polymethylsilsesquioxane added is increased above 2.0 wt %, it is therefore preferable that the addition amount of the polymethylsilsesquioxane be greater than 0.5 wt % but less than 2.0 wt %.
There is also as shown the strength when the polymethylsilsesquioxane is changed into SiO2 in FIG. 2. However, it is not necessary for the preform in accordance with the present invention to be heated so that the polymethylsilsesquioxane is changed into SiO2.
Further, though having made experiments on binders of the silanol group other than polymethylsilsesquioxane, the same result is obtained in the relationship between the addition amount of the binder and the strength of the preform obtained. Therefore, it is preferable that the amount of binder of silanol group to be added is greater than 0.5 wt % but less than 2.0 wt %.
Here, as the forming method of the preform, for example, press-forming or blow-forming is appropriate.